1. Field of the Invention
The present invention relates to a steering wheel and, more specifically, to a structure for mounting an air bag module on a steering wheel with a spoke extending downward from a steering shaft therein.
2. Description of the Related Art
An air bag module usually includes main components therein such as a folded air bag, and inflator for generating a gas and an igniter which form a single unit. It has been well-known that such air bag module is mounted on a steering wheel structure.
The steering wheel structure is mounted on a steering shaft having an inclination with an acute angle to the vertically standing driver's body. When the vehicle is subjected to an impact rearward at the front thereof, the air bag is inflated to contact the driver's body moving forward relative to the steering wheel structure. In this case, the driver's body tends to turn forward. This forward movement of the driver in addition to the inclination of the steering wheel structure causes the steering wheel to deflect forward at its upper half portion and rearward at its lower half portion.
This deflection of the lower half portion of the steering wheel reduces the distance between the steering wheel and the driver so that a shock absorption effect of the air bag module will be deteriorated for the driver's body portion, specifically the abdomen and breast.
In this respect, Japanese Patent Public Disclosure (JP A) No. 62-234762, laid open to the public in 1987, discloses a steering wheel structure providing the upper half portion with a greater stiffness than the lower half portion thereof so that the lower half portion is readily deflected, absorbing an impact energy, when subjected to a forward force by the driver. Japanese Utility Model Public Disclosure (JP U) No. 2-66368, laid open to the public on May 18, 1990, discloses a similar structure of the steering wheel with an air bag module.
It should be noted, however that it is difficult to deflect the steering wheel properly when the structure is subjected to a forward force.
Meanwhile, in order to provide a broader space in the upper portion of the steering wheel for improving the operability and visuality for the instrument panel, it has been proposed that the steering wheel structure have a spoke extending downward from the steering shaft for carrying a steering wheel ring. For this purpose, when three spokes are provided for carrying the steering wheel ring instead of four spokes, two of the spokes extend obliquely downward in a symmetrical relationship with each other and the rest of one spoke extends straight downward. Since these spokes contribute to increase the rigidity, the lower half portion of the steering wheel structure is greater than the upper half portion in stiffness.
Accordingly, it is difficult to form a steering wheel structure which can be displaced forward desirably so as to absorb the impact energy when the driver hits the inflated air bag forward resulting from an impact at the vehicle front.